1. Field of the Invention
The field of the present invention relates generally to valves having seat assemblies and more particularly to a gate valve having pressure responsive seat assemblies.
2. Background
Gate valves are well known for use in controlling flow of fluids. Generally, gate valves are formed by a valve body having a removable bonnet portion and forming inlet and outlet flow passageways connected by an enlarged central chamber. Movably disposed in the enlarged valve chamber is a gate flow closure member that is reciprocated between open and closed positions. A rotatable stem means, normally mounted with and protruding through the valve bonnet, is operably connected to the gate member to effect the desired reciprocating movement. When moved to the closed position, the gate sealingly engages annular seat rings carried by the valve body to block flow of fluid between the flow passageways. When the gate is reciprocated to the open position, the gate is moved from adjacent the seats into the valve chamber for enabling fluid communication between flow passageways through the valve chamber.
One known type of gate valve is the through conduit version. In this type of gate valve the parallel sealing faces of the movable gate flow closure slab or member continuously seal with annular seats carried by the valve body in both the open and closed positions. The parallel sided gate is provided with a flow opening or port that is aligned with the valve flow passageways when in the open position. Because of this construction a straight through flow path or conduit, free of fluid turbulence in the valve chamber, is formed internally of the gate valve which produces the common name used to designate or identify a through conduit type gate valve.
Gate valves are susceptible to a condition known in the trade or art as "pressure-lock" of which through conduit construction are vulnerable in both the open or closed position. Fluid pressure leakage past the valve seats can be trapped in the valve chamber with the result that gate member is pressure locked against operating movement. To return the valve to normal operation, the valve chamber fluid pressure must be vented. This condition may also occur in other forms or types of gate valves.